As a damper there has been proposed is a damper having a hydraulic damper and an actuator for imparting a propelling force to a piston rod of the hydraulic damper, as is disclosed in Japanese Patent Laid-Open Publication No. 2001-180244. According to this proposed damper a rod of the hydraulic damper is formed in a tubular shape and a female screw is formed on the inner periphery side of the rod. Further, a shaft connected at one end thereof to a rotor of a motor and at an opposite end thereof to a male screw member threadably engaged with the female screw of the rod is inserted into the rod of the hydraulic damper, and the piston rod of the hydraulic damper is constituted by the shaft and the rod.
In the proposed damper, a force generated when the shaft and the rod are relatively moved axially by the motor to extend or retract the piston rod is added to a damping force generated in the hydraulic damper, that is, motor torque is converted to a force acting in the direction of the relative movement between the shaft and the rod, thereby causing the said force to act additionally on the damping force of the hydraulic damper to damp oscillation.
In Japanese Patent Laid-Open Publication No. H08 (1996)-197931 there is disclosed a damper comprising a coil spring which resiliently supports a vehicle body side, i.e., a sprung member side of a vehicle, a screw shaft threadably and rotatably engaged with a ball screw nut connected to an axle side, i.e., an unsprung member side, a motor connected to one end of the screw shaft and supported by the sprung member resiliently by interposition between a pair of springs, and a hydraulic damper fixed to the vehicle body side to damp vertical oscillation of the motor. A relative movement between the vehicle body and the axle is controlled actively with rotating torque generated by the motor.